Fix volume level on Web (#838, #862)

[bloemy7]

This fixes the volume level callback on Web. The file was correctly fixed and merged in the web folder in the js folder, but not in the src folder. This simply copies it over. I've tested and it solves the issue.

Resolves #838
Resolves #862

[bloemy7]

@Larpoux could this get merged in please? Thank you.

[Larpoux]

Hi @bloemy7 👍
Thank you very much for your contribution.
Your PR is now merged inside Flutter Sound 9.4.7

I agree that the JS scripts are not clean.
We have duplicates scripts at three (!!!) different places :

• Flutter_sound_core/web/js/flutter_sound
• Flutter_sound_web/js/flutter_sound
• flutter_sound/example/web/flutter_sound

This is dirty.
I remember that I duplicated the scripts into flutter_sound/example: It was not easy to do symbolic links between flutter_sound_core and flutter_sound/examples because it is two different git repo. The problem is when I update the js scripts during an example run (during debug) chrome browser updates the example scripts and not the flutter_sound_core scripts.
I do not remember why it is also duplicated inside flutter_sound_web.

I am going to clean all that. At least we must give an error during bin/new-version.sh if the three directories are not synchronized. But I would prefer to remove those duplications.